“Futaba trainer cord, micro to micro” cable from
www.towerhobbies.com is a workable solution. The ends
of this cable look like Figure 2.

This pin-out for the connector is available in a number
of places on the Web. Figure 3 shows a graphic of the
trainer jack as seen from the back of the Futaba
transmitter.

You are interested in pin 6 “Signal IN;” this is where
you will be injecting your signal to control your airplane. At
the minimum, you will need pin 2 “Ground” and pin 6.
There is some discussion that if you connect pins 4 and 5
that the radio will stay on, but the RF (radio transmitter) will
turn off. Sometimes you will need to send power down the
shorted pins 4 and 5 at the same level you measure at pin
4 when your radio is on to get into this mode. Regardless,
you don’t want this so don’t short those pins. Just turn your

Figure 4. Radio pulse train.

radio on normally. Using an oscilloscope, look at the signal
coming out of pin 3 to find out what your signal voltage
levels are. You will want to inject the same voltage logic
into pin 6 from your controller board.

What about that signal? You have no doubt seen any
number of articles that describe the formation of a hobby
servo control signal. The PPM signal that you will see
coming out of pin 3 will look very similar to Figure 4, but
different. This signal comes from my Futaba six-channel
radio. These pulses are the various channels from 1 to 8
(yeah, 8, go figure) that I can control. Their timings
represent the pulse values that will be sent to the hobby
servos at the receiver after they are encoded in the radio
transmission. Something that is not obvious is that these
pulses are generated with a negative voltage with respect
to ground. Remember that, when you generate your own
signals. Also, the output voltage is about - 4.2V. Play with
your transmitter sticks and see how that changes the pulse
timings. This will be essential for you to understand to
create your own signals.

Now you know how to control your R/C aircraft from
your PC (through your embedded controller board and
trainer cable). This is just the tip of the iceberg, though!
You will need to derive a program for your PC to
intelligently control your aircraft and get heading, speed,
and orientation data back from your plane. It sounds like a
fun project. Please let us all know how it turns out!

Q. Can you suggest a supplier for encoders to be retrofit to stepper motors (NEMA 34)? Thanks.

— dk

A. There are a number of suppliers, but you would have to question their sales staff to be sure their mounting bolt patterns are compatible. Here are a
few suggestions:

www.applied-motion.com or www.mclennan.co.uk

These companies seem to want to sell stepper/encoder
combinations, but also have encoder add-ons. It isn’t clear if
they can be retrofitted.